The terrestrial rotation from the west to the east around its axis has been accelerating since 2020, slightly shortening the duration of the day: it would not be missing from shorter day of 2025record that could fall on Wednesday July 9thTuesday July 22nd or Tuesday August 5th According to the forecasts of the International Earth Rotation and Reference System Service (IERS). In particular, on 9 July the rotation motion of the earth could be shorter than 1.30 milliseconds Compared to the 24 hours of the average calendar day (exactly 86,400 seconds), on 22 July it could arrive at –1.38 milliseconds and on August 5 even a –1.51 milliseconds. Why these three dates? Because in those days the moon will find itself at a greater angular distance from the plane of the terrestrial equator, thus offering less “gravitational resistance” at the speed of terrestrial rotation.
We are talking about milliseconds, that is, imperceptible differences in our daily life. There will therefore be no direct consequences for us, but the question still has its importance because it can have an impact in areas such as the GPS navigation and the big computer networks on which, for example, the finance or the computer security.
The earth is accelerating its rotation and the days are shortened: the data of the last few years
To understand the question we need some context. Our civil calendar is based on days of the same duration of 24 hours “split”. This is the duration average of a complete rotation of our planet, but the exact duration of the day is not so regular: it has small daily variations (within an interval of about 3 milliseconds) due to various factors such as the gravitational disturbances of the moon, the distribution of the mass inside the earth but also in the oceans and in the atmosphere.
If we look at the last two centuries, The day is slowing down on average by 1.7 milliseconds every 100 years Especially because of the Mareal interaction with the moon. In the last decade, however, we are witnessing a mysterious shortening of the day. Since 2020, above all, we have been inaneing record on record: first –1.46 milliseconds in 2020, then –1.59 milliseconds in 2022 and finally –1.66 milliseconds on 5 July 2024which is currently The shortest day ever recorded. It therefore does not seem that in 2025 we will beat this record, but the fact remains that the days lately they continue to be shorter than the average.
Even if we go and watch the average duration of the day Over the course of a year, we note that the years after 2020 – with the exception of 2024 – were shorter than the exact 24 hours, as we see in this table (data source: IERS).
| Year | Duration of the day |
|---|---|
| 2016 | +1.34 milliseconds |
| 2017 | +1.03 milliseconds |
| 2018 | +0.69 milliseconds |
| 2019 | +0.39 milliseconds |
| 2020 | 0.00 milliseconds |
| 2021 | –0.18 milliseconds |
| 2022 | –0.25 milliseconds |
| 2023 | –0.08 milliseconds |
| 2024 | +0.24 milliseconds |
The forecasts for 2025 indicate that this year too it will be “under media” from this point of view. There are various hypotheses to explain this countertrend, but we still don’t have a definitive explanation. What we know is that, if this trend continues, we can have problems ad Align civil time (i.e., to be clear, the one measured with atomic watches) with the astronomical time (the time actually used by the earth to rotate on itself). It might seem like an ultra-technic and irrelevant question, but in reality it has unexpected consequences that can also reach our everyday life.
The problem of the terrestrial rotation that accelerates: the “intercalary seconds”
The current legislation establishes that as soon as the astronomical time “delay” of at least 1 second compared to civil time is introduced a second intercalary (in English Leap Second), i.e. a minute of 61 seconds that the two counts of the time is realized. The possible “slots” are two per year: on 30 June and 31 December at 11:59 pm. The last second intercalate was introduced the 31 December 2016and as long as the terrestrial rotation continues to accelerate there will be no need. Indeed, if the trend does not stop we may have to get to Remove 1 second – Instead of adding it – to compensate the accumulation of the advance of astronomical time on civil time.
In the meantime, in response to this unexpected acceleration, in 2022 The General Conference of Weights and Measures – the body that regulates the units of measurement and their management – proposed to Avoid introducing the second courses for the next 100 years and refer only to the time measured by atomic watches. Among the reasons for this proposal there is the fact that the second intercalands can create malfunctions to all those technological systems that need very precise synchronizations, including the GPS network and the computer networks on which world finance operateswith potential repercussions on telecommunications, transport and cybersicacy.
It is a purely astronomical question that concerns an imperceptible effect to our senses, however has a concrete impact on our society. This is because today as never before Civilization stands on time And, thanks to atomic watches, we are able to measure it more precisely than the limited regularity of astronomical motions.
Will the earth continue to accelerate?
In any case, it is still early to know how the question will evolve. The astronomer Leonid Zotovfor example, is of the opinion that the duration of the day has already reached its minimum and the earth will return to decelerate in the near future. Zotov’s forecast is rightly cautious, because we do not know precisely the mechanisms that influence the duration of the day and that have made the earth accelerate in recent years.
Second Nick Stamatakosmember of the Iers’ board, currently we are not able to predict the duration of the day with more than 6 months – maximum one year – in advance. All that remains is to be able to see.
